First, make sure your username and password are correct.
If this is not the problem, go to Gmail's website and click on the link: "I cannot access my account".
You are then presented with a question: "What's preventing you from accessing Gmail?"
Choose "I forgot my password".
Then click "password recovery page".
You'll be sent to another page where you should enter your username. Type it and hit Submit.
Then Gmail will ask you a secret question you choose when you registered, and that's it.
You'll be able to access you account.